In high tension power transport networks, it is conventional to interchange information between stations situated at the ends of the power transport lines via said lines, in particular for controlling apparatus at the stations and for signalling faults.
Information is often interchanged in telegraphic form by manipulating the amplitude, the frequency, or the phase of signals at predetermined frequency, in particular using the techniques known by the following sets of initials (in English): ASK, FSK or MFSK, and PSK or DPSK. The number of predetermined frequencies varies depending on the technique used, and may be one frequency only in the simplest of cases.
For various reasons, the predetermined frequencies used for conveying information are generally selected to lie in the telephone band and they are used to modulate high frequency carrier waves which are conveyed by the power transport lines. By selecting the telephone band, it is also possible to transmit these frequencies over a telephone connection, should that be deemed desirable. The transmission of information over power transport lines is liable to take place under highly adverse conditions if the transport lines are disturbed by interference or by electrical faults.
However, it is necessary for teleprotection equipment that is intended to receive such information to be as reliable as possible under the worst possible conditions if they are to perform their function, in particular if they are to control protective apparatuses.
This implies, inter alia, a very high degree of recognition reliability in the telegraph signal receivers for receiving the transmitted information, and consequently highly effective frequency filtering in order to extract useful signals from the interferring noise in which they are embedded. Attempts are therefore made to implement matched filters in order to eliminate noise and to augment signals at one of said predetermined frequencies.
In addition, it is desirable to be able to set up banks of identical filters which are tuned to different frequencies, or to be able to modify the tuning of the filters in a simple manner on request in order to enable users to select the predetermined frequencies that they need without requiring the structure of the filters to be modified, as is required when the filters are implemented using conventional analog techniques.
To this end, the present invention therefore proposes a method of implementing programmable matched filters, in particular for a telegraph signal receiver in teleprotection equipment, and it also proposes the corresponding filters and banks of filters.